1. Field of the Invention
The present invention relates to a dispersion-compensating module which is disposed in an optical transmission line suitable for optical communications such as wavelength division multiplexing (WDM) transmission and the like and which, constituting a part of the optical transmission line, compensates for the chromatic dispersion in the wavelength band of WDM signals.
2. Related Background Art
In general, conventional optical communications systems have a configuration in which a transmission optical fiber is mainly used as an optical transmission line, while optical amplifiers are disposed at appropriate repeating intervals. Since signal light attenuates while propagating through the transmission optical fiber, the optical amplifier is employed as an optical device for amplifying the signal light power of WDM signals containing a plurality of wavelengths of signal light components, and the like, for example. The optical amplifier usually comprises an amplifying section for amplifying the optical power of respective light signals and an equalizing section for lowering the gain differences occurring among the respective light signals, thus yielding not only an optical amplifying effect but also a gain-equalizing effect. Therefore, in the case where WDM transmissions are carried out, the optical amplifier can optically amplify the respective light signals of the WDM signals with a substantially uniform gain.
As the transmission optical fiber, on the other hand, a single-mode optical fiber is usually employed. While this single-mode optical fiber has a positive chromatic dispersion (about +17 ps/nm/km) in a 1.55-/xcexcm wavelength band (1500 nm to 1600 nm), if a large chromatic dispersion exists, then the pulse waveform of the WDM signals may deform, thereby causing reception errors. In particular, the existence of chromatic dispersion becomes a large problem if a higher speed, i.e., higher band, on the order of several gigabits/sec to several tens of gigabits/sec is attained. Hence, there has been proposed an optical communication system in which a dispersion compensator is disposed in the optical transmission line in order to compensate for the above-mentioned chromatic dispersion (see, for example, the Institute of Electronics, Information and Communication Engineers, Communication Society Convention 1997, B-10-70 and B-10-71). Also, as the dispersion compensator, a dispersion-compensating optical fiber having a large negative chromatic dispersion (about xe2x88x9290 ps/nm/km) in the 1.55-xcexcm wavelength band is used, for example.
The inventors have studied the above-mentioned prior art and, as a result, found problems as follows. Namely, since deviations in transmission loss among respective light signals in the WDM wavelength band are so large that wavelength dependence is not negligible, there are problems as follows.
By way of example, a typical configuration in which, as shown in FIG. 1A, a dispersion compensator 3 is disposed between an optical amplifier 1 and an optical amplifier 2 in a stage next thereto in an optical transmission line will be explained. In this optical transmission line, the respective light signals (wavelengths xcex1 to xcex4) in the WDM signals outputted from the optical amplifier 1 are assumed to have an uniform optical power.
In the foregoing configuration, when signal light components shown in FIG. 1B are inputted to the optical amplifier 1, then amplified signal light components shown in FIG. 1C are outputted therefrom. As the light signals outputted from the optical amplifier 1 are inputted to the dispersion compensator 3, the chromatic dispersion of the transmission optical fiber is compensated for. On the other hand, since the transmission loss in the dispersion compensator 3 changes depending on a wavelength, the light signals outputted from the dispersion compensator 3 would not have an uniform optical power (see FIG. 1D). Also, if the light signals outputted from the dispersion compensator 3 are further inputted to the optical amplifier 2, the light signals outputted from the optical amplifier 2 (see FIG. 1E) will have been amplified in a state including the optical power differences among the respective light signals at the time of input. Therefore, in the case where a plurality of dispersion compensators are disposed between a transmitting station and a receiving station, the differences in optical power among the light signals reaching the receiving station would become greater as they are successively accumulated. If the differences in optical power among the light signals reaching the receiving station are large as such, some signal light components may deteriorate their S/N ratio so much that they cannot be received. Hence, in the case of inserting a dispersion compensator, it is necessary to redesign an optical transmission system as a whole, so as to eliminate the above-mentioned problems.
For solving the above-mentioned problems, it is an object of the present invention to provide a dispersion-compensating module which functions to compensate for the chromatic dispersion occurring in an optical transmission line and which has a low wavelength dependence of transmission loss, having its structure adapted to be easily inserted in an optical transmission system.
Therefore, the dispersion-compensating module according to the present invention has an input end on which light of one or more light signals (included in WDM signals) whose respective center wavelengths are included in a predetermined wavelength band are inputted, and an output end from which the WDM signals exit; and can be installed not only between a transmitting station and a receiving station, but also between the transmitting station and a repeater, between repeaters, and between a repeater and the receiving station. Also, the dispersion-compensating module comprises dispersion-compensating means, such as a dispersion-compensating optical fiber or the like, disposed in an optical path between the input end and the output end in order to lower the wavelength dependence of transmission loss; and loss-equalizing means for compensating for a wavelength-dependent loss deviation of the dispersion-compensating means.
Here, depending on the object to be compensated for, the above-mentioned dispersion-compensating means has a positive or negative dispersion slope in the wavelength band of the above-mentioned WDM signals (e.g., 1500 nm to 1600 nm). Further, depending on the object to be compensated for, the dispersion of the dispersion-compensating means has a positive or negative value in the wavelength band of the WDM signals.
Also, the dispersion-compensating module according to the present invention can function as a repeater when further comprising optical amplifying means. In this configuration, the above-mentioned loss-equalizing means compensates for at least the loss deviations of the above-mentioned dispersion-compensating means depending on the wavelengths of respective light signals and the gain deviations of the above-mentioned dispersion-compensating means dependent on the wavelengths of respective light signals.
Further, the dispersion-compensating module according to the present invention can further comprise a demultiplexer for demultiplexing each of the above-mentioned signal light components, and a multiplexer for multiplexing respective light signals demultiplexed by the demultiplexer. In this configuration, the above-mentioned dispersion-compensating means compensates for a dispersion in a larger wavelength band between the entrance end and the demultiplexer, and also compensates for a dispersion in a smaller wavelength band for respective demultiplexed light signals. The above-mentioned loss-equalizing means adjusts the optical power of respective demultiplexed light signals. Preferably, the loss-equalizing means is disposed in an optical path between the entrance end of the dispersion-compensating module and the dispersion-compensating compensating means, i.e., in front of the dispersion-compensating means in the propagating direction of respective light signals. In this case, since the light signals inputted to the dispersion-compensating module are inputted to the dispersion-compensating means after being attenuated by their desirable values corresponding to the respective wavelengths thereof by the loss-equalizing means, nonlinear optical phenomena are unlikely to occur in the dispersion-compensating means, whereby the light signals are kept from deteriorating their waveforms. Also in this configuration, while the chromatic dispersion in the optical transmission line is compensated for by the dispersion-compensating means appropriately disposed at a predetermined location, the wavelength-dependent loss deviation of the dispersion-compensating means is compensated for by the loss-equalizing means for adjusting the optical power of respective light signals between the input end and the dispersion-compensating means.
Specifically, the above-mentioned loss-equalizing means may be a loss-equalizing optical fiber having a core region doped with a transition metal, and a cladding region disposed on the outer periphery of the core region. By appropriately selecting the kind and amount of transition metal such as Cr element, Co element, or the like added into the core region, such a loss-equalizing optical fiber is easily designed so as to compensate for the wavelength-dependent loss deviation of the dispersion-compensating means.
Also, the above-mentioned dispersion-compensating means includes a single-mode optical fiber having a zero-dispersion wavelength in a 1.3-xcexcm wavelength band or a dispersion-shifted optical fiber, whereas the above-mentioned loss-equalizing means includes an optical fiber formed with a long-period,fiber grating which couples a propagation mode and a radiation mode. This long-period fiber grating is an optical component which is clearly distinguished from a short-period fiber grating which reflects only a predetermined wavelength of signal light component. Such a long-period fiber grating acting as the loss-equalizing means can flatten the loss deviation among the respective light signals without greatly deteriorating the transmission loss of the dispersion-compensating module as a whole, and can easily yield a desirable loss characteristic in a wide wavelength band. In particular, in the configuration mentioned above in which the optical fiber acting as the dispersion-compensating means is directly formed with the long-period fiber grating acting as the loss-equalizing means, it is not necessary for the dispersion-compensating means to be provided with a connecting portion which may generate loss, and the influence of the loss in the connecting portion is not needed to be taken into consideration, whereby it becomes easier to adjust wavelength-dependent loss characteristics.
Further, the above-mentioned loss-equalizing means may be a fiber fusion type coupler (fiber coupler). In particular, it is preferable for this fiber coupler to have a polarization-dependent loss (PDL) of 0.2 dB or less. It is because a fiber coupler having a PDL greater than 0.2 dB cannot strictly control the compensation of the PDL.
On the other hand, the above-mentioned loss-equalizing means may be made of a fused portion obtained by fusion-splicing respective ends of a pair of optical fibers. In this case, the pair of optical fibers at the fused portion may be fusion-spliced in a state where their respective optical axes are deviated from each other or in a state where their core regions are bent. Further, each of the pair of optical fibers to be fusion-spliced may comprise a core region whose diameter increases toward the fused portion. In any case, a desirable characteristic (characteristic with a smaller wavelength dependence) can favorably be obtained.
Preferably, in the dispersion-compensating module according to the present invention, of the light signals emitted from the above-mentioned exit end, at least those having their center wavelength within the wavelength range of 1530 nm to 1565 nm have an optical power deviation of 0.5 dB or less therebetween. It is because of the fact that favorable transmission characteristics can be expected over several hundreds of kilometers if the optical power deviation among the light signals is suppressed to the above-mentioned value or less in a wavelength band used in a normal erbium-doped fiber amplifier (EDFA).
Also, in the WDM transmission in a 1580-nm band, of the light signals emitted from the above-mentioned exit end, at least those having their center wavelength within the wavelength range of 1560 nm to 1600 nm preferably have an optical power deviation of 0.5 dB or less therebetween. It is because of the fact that favorable transmission characteristics can be expected over several hundreds of kilometers if this condition is satisfied.
In particular, in the long-distance optical transmission beyond 1000 km, of the light signals emitted from the above-mentioned exit end, at least those having their center wavelength within the wavelength range of 1535 nm to 1560 nm preferably have an optical power deviation of 0.5 dB or less therebetween; and further, of the light signals emitted from the above-mentioned exit end, at least those having their center wavelength within the wavelength range of 1575 nm to 1595 nm preferably have an optical power deviation of 0.5 dB or less therebetween.
For yielding a favorable transmission characteristic with a BER (Bit Error Ratio) of 10xe2x88x9215 or less in the high-speed transmission of 10 gigabits/sec or faster in the long-distance optical transmission beyond 1000 km, on the other hand, of the light signals emitted from the above-mentioned exit end, at least those having their center wavelength within the wavelength range of 1550 nm to 1560 nm preferably have an optical power deviation of 0.2 dB or less therebetween; and further, of the light signals emitted from the above-mentioned exit end, at least those having their center wavelength within the wavelength range of 1575 nm to 1585 nm preferably have an optical power deviation of 0.2 dB or less therebetween.
The present invention will be more fully understood from the detailed description given hereinbelow and the accompanying drawings, which are given by way of illustration only and are not to be considered as limiting the present invention.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will be apparent to those skilled in the art from this detailed description.